(1) Field of the Invention
The present invention relates to the general technical field of mechanical damping, and more particularly to the technical field of equipment used in aviation and including damper systems.
The invention is applicable in any damper system when a loss in damping characteristics might lead to a catastrophic event. The invention thus applies most particularly to lead-lag dampers used on a helicopter rotor, e.g. a main rotor or an anti-torque motor.
(2) Description of Related Art
By way of example, on a helicopter rotor that is said to be “soft in-plane”, a lead-lag damper system connects each blade to the hub of the rotor or connects together two consecutive blades.
Such a damper serves to avoid catastrophic coupling between the lead-lag movements of the blades and the helicopter airframe. These phenomena are known as “ground resonance” and as “air resonance”.
Such a damper is subjected to forced movements at the frequency of rotation of the rotor due to the pitch, flapping, and drag movements of the blade. The damper has very little effect on these movements.
The damper is also subjected to movements at the lead-lag resonant frequency of the blade. These movements are not naturally damped, e.g. by aerodynamic forces. Consequently, the damper serves to oppose any resonant phenomena.
It is essential to ensure that such a damper or damper system is in a good state of operation.
The present invention thus relates to means for ensuring that the damping characteristics of the damper system are indeed capable of being satisfactory. The invention thus relates to viscoelastic dampers and also to other types of damper, e.g. hydraulic dampers, by providing means for monitoring the absence of any hydraulic fluid leakage that would be harmful to proper operation of said dampers.
It is known, in particular from document FR 2 867 152, to control a lead-lag damper by adjusting its damping characteristics as a function of measured vibration so as to optimize energy dissipation. Such a method is dedicated to specific dampers and is technically complex. In addition, such a method is not designed to identify a loss of damper effectiveness.
Also known, e.g. from document U.S. Pat. No. 3,080,803, is a bench for measuring crack propagation in a test piece made of an elastic material. Crack monitoring is performed by special lighting and by making use of images obtained by means of a camera. The operation consists in periodically measuring crack size and is time consuming, and it also requires a large amount of know-how on the part of an operator.
Document U.S. Pat. No. 4,574,642 discloses a method that characterizes an elastomer by analyzing the movements of a mass in a vibration mode. Such a method serves to determine the stiffness and the damping of an elastomer once the excitation frequency and the excited mass are known.
With known methods, when the observed cracks reach a certain size, the damper is replaced. That generally requires the damper to be dismantled during a maintenance operation.
Consequently, those methods are not suited to helicopter rotor dampers since the forces to which the blades are subjected and the corresponding frequencies are not accurately identifiable. Those methods are also not applicable to rotor-mounted dampers or damper systems while they are in operation.